1. Field of the Invention
This invention relates to an optical system for monitoring the optical quality of solar cells, and more particularly to a reflectance measuring system for use in monitoring the surface texture, metallization, and anti-reflective coating of solar cells in commercial production.
2. Description of the Prior Art
In the fabrication of photovoltaic cells it is necessary to precisely control the sawing, cleaning, texturing, dielectric-film-coating, and metallization process steps. Texture-etching is used to improve the light trapping ability of a solar cell, by reducing the surface reflectance over a broad light spectrum. Texture-etching is also used to remove any saw damage to the surface of a cell. Deposition of a dielectric-film layer, over the textured surface, is used to further reduce reflectance. Metallization of the cell includes alloying an aluminum back contact, and screen printing a front metal contact to the cell. Any failure to tightly control these process steps lends itself to the fabrication of devices, which exhibit a variance in the light-trapping ability, photo-current, fill-factor, and the open-circuit voltage of the cell. Accordingly, there is a need for an optical system useful in the quality control of these process steps.
Various optical systems are available for monitoring the physical characteristics of a photocell. In the prior art, many of these systems measure sample reflectance in a light integrating sphere. The light reflected from the sample is measured spectroscopically. For example, as described in U.S. Pat. Nos. 4,932,779, and 5,406,367, for color measurement, an integrating sphere is provided to receive light, from a light source, through an entrance port. The diffusely reflecting interior walls, of the sphere, reflect the light in multiple reflections, such that a uniform diffuse illumination is provided over the interior surface of the integrating sphere. The integrating sphere is provided with a port designed to receive a sample, the color of which is to be measured. When a sample is positioned over the sample port, the surface of the sample is illuminated with uniform diffuse illumination, reflected from the walls of the integrating sphere. An exit port is located on the sphere, opposite the sample port, for receiving diffusely reflected light from the sample, and the light passing through the exit port is separated into monochromatic components. The intensities of the components are measured, to determine the reflectance of the sample, for each monochromatic component. However, as demonstrated in the foregoing patents, an integrating sphere is used to analyze a small sample area, because the sample, itself, disrupts integration of the illumination. Because commercially sized samples have a large, 4 by 4 inch, surface, it would be necessary to provide an unreasonably large integrating sphere, to rapidly monitor the surface area of a solar cell.
In Sopori, B. L., Principle of a New Reflectometer for Measuring Dielectric Film Thickness on Substrates of Arbitrary Surface Characteristics, Rev, Sci. Instru. Vol. 59 no. 5 (May, 1988), pp. 725-727, a reciprocal optical principle, and a relative small light-absorbing sphere, has been used to determine the thickness of an antireflection film, layered over a silicon cell. The reciprocal principle is based on the projection of incident light, at a wide solid-angle of direction, at the sample surface and detecting the intensity of a reflection, normal to the sample surface. The reflectometer comprises a metallic, spherical, dome having openings for two ELH-tungsten-halogen lamps and elliptical reflectors. One lamp is located on each side of the dome. An exit aperture and lens assembly is located at the top of the dome for emitting the reflection. At the base of the dome, diametrically opposed to the exit aperture is a highly absorbing sample support. The support is covered with small-grain polycrystalline sheets, etched and layered with a Si.sub.3 N.sub.4 deposit, in order to reduce reflectance. Located at the top of the dome is a monochromator and detector, connected to a display device. The display generates a reflection intensity distribution curve for the reflection. The reflectance of a textured sample, having an antireflection coating, exhibits a minimum intensity, on the curve, which is useful in determining the thickness of the film, according to the equation: t=.lambda..sub.0 /4.sub.n, where .lambda..sub.0 is the wavelength having a least reflectance, t is the thickness, and n is the refractive index of the film.
The absorbing and light integrating spheres are similar in construction, but the absorbing sphere must function to eliminate all, extraneous, scattered light. In doing so, the normal reflection is the only light detected. The major extraneous light-scattering source, in an absorbing sphere, is the cell support. While etching a fine grain polycrystalline silicon wafer and depositing a layer of Si.sub.3 N.sub.4 has produced a non-reflecting support, the monitoring system, according to this invention, provides a significantly different light absorbing baffle and a non-reflecting chuck in lieu of the silicon wafer support. Other significant differences are also included. This invention provides for an increase in the spectrum of projected light in order to generate a reflection from the back-side-contact, is able to monitor the area, thickness, and symmetry of a front-contact, and, because some venders produce cells having a specular surface, is able to monitor the characteristics of a cell having a polished or specular finish. These improvements are desirable in a system, which is useful, to monitor the texture, antireflective film, and metallization properties of solar cells in commercial production.
Thus, in view of the foregoing considerations, there is an apparent need for an optical system which is cost efficient, versatile in use, and capable of arriving at a rapid precise determination of the texture, metallization, and dielectric film optical properties of solar cells in commercial production.